IE will not submit unless file field is correctly specified

When a file upload field has an incorrectly specified file, browsers respond in different ways.

IE solves the problem by preventing form submission.
This approach is reasonably sensible, however it would be much better if IE popped up a warning dialog saying "File not found".

Firefox takes a totally different approach; it lets the server sort out an appropriate response.
It means I have to code the server for this use case (good practice anyhow) and I can put up my own warning messages.

In summary, the Firefox approach needs a bit more coding but gives me complete control over the page. IE means well, but leaves my users confused and unable to leave the page.